(1) Field of the Invention
This invention relates to a method for heating a semiconductor wafer by means of application of radiated light.
(2) Description of the Prior Art
The ion implantation process has recently been finding actual utility as an effective method for introducing dopant atoms into a semiconductor wafer (hereinafter "wafer" for the sake of brevity) since it is possible to control accurately the concentration levels of dopant atoms and the depths of resulting junctions. In the ion implantation process, the dopant atoms are ionized and accelerated to high velocity and the wafer is bombarded with the dopant atoms. Whenever the ion implantation process is carried out, it is necessary to subject each resulting wafer to a subsequent heating treatment at about 900.degree. C. or higher so that any crystal defects which have been developed due to changes in crystalline state at the surface of the wafer can be healed to ensured desired surface conditions. This heat treatment must be carried out in a short time period so as to prevent the concentration distribution of the implanted dopant atoms in the depthwise direction of the wafer from changing due to thermal diffusion. Furthermore, there is an outstanding demand for the establishment of a high-speed heating and cooling cycle for wafers in order to improve their productivity.
Reflecting the above-mentioned demands, a novel method has recently been developed to heat wafers by means of application of radiated light. According to this method, the temperatures of wafers may be raised to some hundred degrees above 1000.degree. C. in a time period as short as a few seconds.
It has however been found that, when a wafer, for example, a wafer of a single crystal of silicon, is heated to a treatment temperature of about 1000.degree. C. or so in a short time period of not longer than a few seconds and is then held at that treatment temperature by means of mere application of radiated light so as to carry out its heat treatment, a relatively large temperature difference occurs between the circumferential portion of the wafer and its central portion in the course of the heat-raising and heat treatment step, whereby the wafer develops such large "warping" as to impair subsequent treatment and/or processing steps and also a damage called "slip line" in the wafer.
The thickness of a wafer is generally very small, namely, of a level of about 0.5 mm or so and its thickness temperature distribution is thus rendered substantially uniform in a very short time period of a level of 10.sup.-3 second or so. Accordingly, a wafer is not practically affected adversely by any thicknesswise non-uniform temperature distribution. The above-mentioned "warping" or "slip line" is hence caused by the non-uniform temperature distribution in the direction extending along the surfaces of the wafer. Specifically, the temperature of the wafer is kept considerably lower at its circumferential portion compared with its central portion, because even if the surfaces of the wafer are heated with a uniform radiant energy density by means of application of radiated light, far more heat is allowed to radiate from the circumferential portion of the wafer than its central portion, and the temperature of the circumferential portion of the wafer cannot thus follow that of the central portion of the wafer when the temperature of the wafer is raised, and the former temperature cannot reach the latter temperature even during the heat treatment of the wafer.
If a wafer develops such large "warping", some problems will be encountered in its subsequent treatment and/or processing steps, including for example that a pattern image may be distorted in the photoetching treatment step. On the other hand, the occurrence of "slip line" makes it impossible to use the wafer as a semiconductor material, in other words, destroys its value and, accordingly, leads to a fatal loss.